Motor vehicles use horns to warn other drivers and pedestrians. The driver typically presses the center part of the vehicle steering wheel to active the horn. Pressing the center part moves a portion of the steering-wheel/airbag cover to close a mechanical switching device. Because the steering wheel also houses an airbag assembly for protecting the driver during a collision, the arrangement for closing the switching device can become complicated. The airbag assembly creates problems because it is arranged between the cover and the switching device for activating the horn.
In this respect, German patent publication DE 197 32 022 discloses integrating the switching device with the airbag cover. Here, the switching device is formed by a contact arranged on the upper edge of a container accommodating the airbag assembly, and an opposite contact arranged on the underside of a web projecting from the airbag cover and directed toward the container edge. The web supports the airbag cover on the upper edge of the container using compression springs. In this arrangement, the driver can no longer directly activate the horn. This arrangement uses a lower switching current for improved electromagnetic compatibility. But with a lower switching current, voltage spikes from metal contacts of the switching device closing can interfere with the operation. That is, when voltage spikes appear, electrical components, such as horns, cannot be reliably activated.
To solve this problem, a piezoelectric contact film has been proposed. The film can be arranged between the airbag cover and the airbag so that the horn can be activated in the usual way by axially pressing the central area of the airbag cover. Examples of this solution are disclosed in EP 534 694 B1, U.S. Pat. No. 5,499,841, DE 43 29 120 A1, and EP 568 764 A1. This solution, however, is relatively expensive because application of the piezoelectric material is relatively expensive.
Other solutions involve incorporating an electronic device for detecting and correcting the voltage spikes. This too is expensive since a sub-assembly is needed. Accordingly, there is a need for a reliable horn activation device without the above-identified drawbacks. The present invention addresses this need.
The present invention relates to a steering wheel assembly and an electrical switching device thereof, which can be used, for example, for activating a horn or any other electrically run devices.
One aspect of the invention is the electrical switching device, which includes first and second electrical contact elements. At least one of the first and second contact elements comprises an electrically conductive synthetic material and a metal conductor at least partially embedded therein, the synthetic material forming a contact surface. Both of the first and second contact elements can be made of electrically conductive synthetic material and metal conductor at least partially embedded therein, with their synthetic material forming contact surfaces of the first and second contact elements.
The synthetic material can be an elastomer, more preferably a thermoplastic elastomer. The synthetic material can contain conductive fillers, which can be carbon fibers. One or both of the first and second contact elements can be extruded or injection molded. One of the first and second contact elements can be formed of a sheet metal. The second contact element can be formed of a sheet metal.
Another aspect of the present invention is the steering wheel assembly, which includes a steering wheel base, a displaceable member displaceable relative to the steering wheel base; and the electrical switching device described above. The first contact element can be arranged on the base, and the second contact element can be arranged on one of the base and the displaceable member. Displacing the displaceable member electrically connects the first and second contact elements. The displaceable member can be an airbag module having a housing for holding an airbag and a gas generator. The housing can be displaceable relative to the steering wheel base. The housing can comprise a carrier for housing a gas generator and an airbag, and a cover connected to the carrier for covering the carrier. The second contact element can be arranged at least on one of the carrier and the cover.
According to one embodiment, the steering wheel assembly can have a third contact element movable with the housing relative to the first and second contact elements. Here, the third contact element can be configured to contact both the first and second conductive elements to electrically connect them. The third contact element can be electrically conductive. In this respect, the second contact element is arranged on the base next to the first contact element, and the housing has at least one conductive portion that contacts both the first and second contact elements.
According to another embodiment, the second contact element can be arranged on the housing and positioned so that at least a portion of the second contact element directly contacts a portion of the first contact element when the housing is displaced toward the base. That is, the second contact element can be displaceably mounted relative to the base so that the second contact element is movable relative to the first contact element. The second contact element can comprise at least one projection integrally formed with the housing. The housing can comprise a carrier for housing a gas generator and an airbag, and a cover connected to the carrier for covering the carrier. The second contact element can be arranged at least on the cover. Moreover, the second contact element can be arranged both on the cover and the carrier. Moreover, the second contact element can be arranged on a peripheral end of the cover. The steering wheel assembly can further include a protective element, such as elastic foam, for isolating at least one of the first and second contact elements from dust.
According to another embodiment, both the first and second contact elements can be arranged on the base. The second contact element can be spaced from the first contact element with a gap, and the housing can have at least one pushing portion configured to push the second contact element toward the first contact element when the housing is displaced toward the base. The switching device also can include a switching-element housing supporting both the first and second contact elements. The switching-element housing can be attached to the base. The switching-element housing can be non-conductive. The pushing portion can contact a portion of the switching-element housing. At least the portion of the switching-element housing that contacts the pushing portion can be elastically deformable.
The first or second contact element or both can be configured to be seated in a groove formed respectively on the base or the housing (either or both the cover and the carrier).